Why 67 Axle is loose by 1/8

[josuito]

hello guys, I'm new here.
I have a 67 c10 step-side when I got it had the 6 lug so I did the 5x4.75 lug conversion front disc and rear (that's what the last owner gave me, me too, I taught it was going to be the 5x5 pattern). I left the same 67 differential, just changed the axles and I think some parts inside. Axle measures 30-3/4"

My mechanic guy said the axles on the differential were loose, you can pull/push them by about 1/8" or 1/4". Also, you can see the drum is not fully covering the whole area where the brake shoes and all that goes.

Now, What can I do there, do I put any washers inside the differential so it's tide or buy a new part? Any help will be appreciated, guys.

Thanks again.